Imports are illegal in the first place. Nobody really enforces the law though, unless they are asked. My guess would be that Sega asked Ebay to not allow Import Dreamcast games, and recently, Sega told Ebay that it's okay for people to sell them on there now.

i was under the impression that ebay decided to ban all import auctions regardless of system (probabbly because they would require modchips etc for some systems (more psx related than sega/dc, since you can use the boot disc on dc for imports or the chip)